This invention relates to ion sources for producing an ion beam. The invention was developed through use with end-Hall effect ion sources and is, at times, described with particular reference thereto. It will be apparent to the skilled reader however, that the scope of the invention will encompass other types of ion sources.
Ion sources had their origins in space propulsion but more recently have found use in more industrial processes such as Ion Assisted Deposition (IAD) of thin film coatings. In an IAD process, an ion beam from an ion source is directed onto a target substrate to cause densification of the coating material as it is deposited. The process occurs within an evacuated chamber of pressure of the order 10−2 Pa.
In a typical ion source, electrons are drawn from a cathode filament toward an anode through an ionizable gas. Collisions between the gas molecules and energetic electrons create a source of positive ions by inducing a plasma. In one type of ion source known as a gridless ion source, a magnetic field is applied across the plasma to shape the ions accelerated from the ion source into an ion beam. In a specific type of gridless ion source, known as an end-Hail effect ion source, the axis of the magnetic field is aligned with the electric potential between the cathode and the anode. The interaction of the magnetic and electric fields causes the charged particles to approximately follow the magnetic field lines. The anode in these devices is typically annular having an outwardly inclined inner diameter with the bulk of the plasma forming within the confines of the anode walls.
An example of an end-Hall effect ion source in common use, in particular in IAD techniques, is described in U.S. Pat. No. 4,862,032 to Kaufman el al. In this device, herein referred to as the Kaufman device, the ionizable gas is distributed uniformly across the plasma region. Magnetic field shaping disperses the electrons across the gas to ensure a large plasma capable of producing a high ion beam current. The result is that a relatively high gas flow (typically up to 50 sccm) is required maintain a sufficient pressure in the plasma region to achieve ionization of the gas. The resultant high background pressure within the interelectrode space creates electrical instability leading to the generation of cathode spots within the ion source and extending to the extremities of the vacuum environment. In addition, large vacuum pumps are required to maintain a sufficiently low pressure within the rest of the evacuated chamber to be compatible with the operation of other equipment used in IAD and other processes. In operation the pressure can only be increased to the point where the ion beam current is approximately 1 Amp before further instabilities are introduce.
A further problem with present ion sources is that their performance can decrease over the life of the ion source. Symptoms include difficulty in establishing the plasma and a reduced stability of the plasma. Investigations by the present inventor have found that the reduced performance capabilities are created, at least in part, by a decrease in the electron flux entering the ionization region due to a reduction in the effective surface potential of the anode. Further investigation into the cause of the reduced potential by the present inventor found that a dielectric oxide layer built up on the surface of the anode exposed to the plasma. It was previously believed that the observed build up of electrically insulating coatings on the anode were produced by scattering and sputtering from the thin film deposition processes for which these ion sources were commonly used. The inventor has found that the dielectric layer actually arises from a small percentage of negative ions produced in an oxygen plasma interacting with the surface of the anode and that this has the effect of shielding the anode from the cathode, dispersing the electron flow from the cathode and thus reducing the electron flux into the ionization region. The reduced electron flux into the ionization region firstly creates instability in the performance of the ion source and, secondly, causes an imbalance in the change neutrality of the resultant ion beam.